For years, it has been standard practice to connect exposed metallic surfaces of electrical gear to ground potential to reduce the probability of such surfaces having a live potential and subjecting personnel to shock hazards. Experience has shown that unless there is good electrical conductivity between a true ground and the exposed metallic surface, that the surface may not be at a true ground potential and that there is possible shock hazard. In addition, experience has demonstrated that if the grounding circuits have the capacity to conduct large fault currents, the extent of damage resulting from accidental short circuits, fire, lightning and other abnormal conditions, may be materially reduced. Accordingly, efforts have been made to increase conductivity from exposed metal surfaces to ground. Frequently, this includes the use of auxiliary conductors. Since these conductors normally carry little, if any, current their efficacy is normally tested only under adverse conditions. In the case of rigid metallic conduit and electrical metallic tubing, the conduit itself has proven over the years to be a good conductor of ground currents and has provided good service in actual field performance. However, in the case of liquid tight flexible metal conduit, some questions have been raised as to the ability of the conduit itself to carry high order of magnitude fault currents. In sizes larger than 11/2 inch, the National Electric Code requires grounding conductors in addition to the current carrying conductors when liquid tight flexible metal conduit is used. In order to provide some means of verifying the validity of grounding conductors, it has become a common practice to run an external grounding conductor along with the conduit so as to provide a more reliable ground path and to have an easily inspectable ground path.
U.S. Pat. No. 3,009,128 discloses what is known in the electrical fittings trade as a grounding bushing which might typically be used for terminating a metal conduit to an electrical panel. The patented grounding bushing has not been a significant commercial success. This may be due to the fact that the lug is not removable from the bushing and in cases where the bushing is installed near the edge of a box; for instance, there is no room to install the bushing. Grounding bushings with removable lugs have been in the industry for years.
United States patent application Ser. No. 623,832 filed Oct. 20, 1975, and issued as U.S. Pat. No. 4,000,918 on Jan. 4, 1977 to Frederick A. Reker and assigned to the same assignee as the present application, discloses a ferrule for making a liquid tight connection to flexible armored conduit. The structure disclosed in application Ser. No. 623,832 finds utility in the present structure.
It is frequently necessary to place electrical apparatus, fittings and enclosures in atmospheres or locations which may be subjected to considerable moisture or liquid. In such situations, it is frequently desirable to provide moisture or liquid tight seals to prevent the entrance of moisture, or liquid, into the electrical apparatus. It has become fairly standard to use various types of gaskets to provide the required moisture barrier. The use of such gaskets has tended to reduce the metallic contact between joined members. The reduced metallic contact may reduce the ability to conduct fault currents through the fitting. This has been a particular problem with liquid tight, flexible metallic conduit which is a marginal conductor of fault current. To overcome this conductivity problem, external ground conductors have been used in addition to the conduit itself.